Metal cord

ABSTRACT

A metal cord ( 100 ) comprising at least two metal strands ( 101  A-G). At least one strand (lOlD) is welded, said welded strand having an elongation at rupture of more than 30% of the elongation at rupture of an identical strand without weld.

The present invention relates to metal cords and more in particular tothe improvement of the tensile strength of the metal cord at weldspresent over the length of the metal cord.

BACKGROUND OF THE INVENTION

At present, using welds to connect two ends of metal cords is avoided asmuch as possible. This is since metal cord welds create an inflexiblezone in the metal cord, disturbing e.g. the bending of the metal cord.Further, the weld in the metal cord creates a weak zone as far asmechanical properties such as tensile strength are concerned. The forceat rupture of a welding zone of a cord is usually less than 60% of theforce at rupture of the metal cord without the welding zone.

Another important aspect is noticed when several metal cords are usede.g. to reinforce a timing belt or for use in elevator belts. When onemetal cord breaks during load, this seems to create a “shock-effect” or“cascade effect”, which usually causes break of all adjacent metalcords, or even the whole belt. The same applies when several windings ofa metal cord are used to reinforce e.g. endless belts. This is anotherreason why no welds are allowed in these applications.

An other disadvantage of the inflexible zone of the weld, using metalcords in polymer or rubber matrices, is that at the welding zone, due tothe inflexibility of the weld, the weld tends to migrate in the polymeror rubber in a transversal direction when the metal cord is bend. Thismay cause shearing of the matrix.

Avoiding the welds of cords is obtained by demanding predeterminedlengths for the metal cords without welds to be supplied. This specifiedlength causes a significant price increase of the metal cords. The metalcord manufacturer faces a waste increase, since insufficient lengths ofmetal cords or intermediate products cannot be sold or used.

SUMMARY OF THE INVENTION

The present invention provides a metal cord, which overcomes or reducesthe problems of price increase and decrease of mechanical properties ofthe metal cord and possibly the object in which the cord is used.

According to the present invention, the metal cord comprises at leasttwo strands. At least one of these strands comprises a weld, whichwelded strand having an elongation at rupture, which is at least 30%,but preferably larger 35% or even more than 40% of the elongation atrupture of the metal strand in which no weld is present.

Preferably, the welded strand has a force at rupture being more than30%, but preferably larger 35% or even more than 40% of the force atrupture of the not-welded strand.

The weld at the metal strand preferably has a diameter, being less than1.2 times the strand diameter or even more preferred, less than 1.1times the strand diameter. The weld at the metal strand preferably has adiameter, being more than 0.8 times the strand diameter or even morepreferred, more than 0.9 times the strand diameter. Most preferably, theweld diameter is essentially identical to the strand diameter itself.

A metal cord as subject of the invention has to comprise at least tostrands, most preferably however, such metal cords comprises more than 2strands, such as 3, 4, 5, 6, 7, 8, 9 or even more strands. All strandsmay be identical, or may be provided using different strandconstructions, a different number of filaments or different filamentdiameters.

Preferably, for each radial cross-section of the metal cord, there is atleast one, but more preferably more than one strand which is not weldedin this cross-section.

Even more preferred, at each radial cross-section of the metal cord assubject of the invention, only one metal strand is welded. At thelocation in the metal cord, where this one metal strand is welded, theother strands of the metal cord are not interrupted and do not comprisea weld near the weld of the one metal strand being welded.

Preferably the other strands do not have a weld over a length of themetal cord (hereafter called “welded part”) which comprises the weld ofthe one metal strand of the metal cord. This welded part has a weldedpart length L, being at least 2 times the lay length of the metal cord.Possibly the welded part length is 2.5 or 5 or even 10 times the laylength of the metal cord.

Such weld having a relatively high elasticity may be obtained by usingcarefully chosen settings during the welding and preparation steps.

A method to provide a weld, which has improved elasticity, may beobtained by:

-   -   Providing an essentially flat cross-section on both ends of the        metal cord which are to be welded to each other;    -   At both ends of the metal strand, to be welded to each other,        decreasing the lay length L_(s) of the strand; L_(s) may be        decreased locally to less than 75% of the original lay length of        the strand, or even less than 65% or less than 50% of the        original lay length of the strand.    -   During welding, the welding parameters are to be set according        to the strand properties; such welding parameters are welding        time, welding current, welding pressure and the distance between        the electrodes fixed on the metal strand; the two ends to be        welded are clamped by two electrodes, at a predetermined        distance of the end of the strand to be welded. This distance is        preferably as small as possible, e.g. less than 2 mm, meanwhile        leaving enough strand material to be welded. Both ends are        pressed to each other with a preset pressure and during a preset        time, a specific electrical current flows through the strand        from one electrode to the other.    -   After welding, the annealing parameters are to be chosen        according to the strand parameters. Such annealing is preferably        done using electrical current. Two annealing electrodes are        fixed to the strand each on an essentially identical preset        distance of the weld, and during a preset time, a preset        electrical current is to flow through the strand from the one        annealing electrode to the other.

The so-called welded part length may comprise at least the length ofboth metal strand ends, which have a decreased lay length, obtained bythe preparation steps. The weld is preferably located essentially in themiddle of the welded part length. The welded part length of the metalcord is then at least equal to the length of the metal cord, whichcomprises both ends with decreased lay length.

The distance between the welding electrodes and the distance between theannealing electrodes is preferably smaller than the sum of the lengthswith decreased lay length.

Such welds have the advantage that they further can be used in theprocess of cord construction operations, either double or singletwisting operations.

A metal cord as subject of the invention may have any cord constructioncomprising at least two strands, each comprising at least two metalfilaments. However, some constructions have a larger benefit of theinvention.

Preferably, ‘n×m-’constructions are used, e.g. 3×3, 7×3, 4×7, 7×4, 7×7or 7×19. n is to be understood as the number of strands in the cord,each strand having m filaments. It is understood that each strand maycomprise different filaments, which are of e.g. a different diameter, adifferent alloy or have a different coating.

Also ‘m₁+(n×m₂)-’constructions may preferably be used. Around a corestrand of m₁ filaments, n strands of m₂ filaments are provided. Anexample is a 3+5×7-cord, being a cord comprising a core stand of 3filaments, around which 5 strands of 7 filaments each are provided. Thefilaments m₁ and m₂ may be of the same or a different diameter. As anexample, a 19+8*7-cord may be provided. The cord has a optical corddiameter of 1.7 mm. Such metal cord is provided using a core strand,which comprises a core filament of diameter 0.19 mm, around which 18filaments of diameter 0.16 mm is twisted in two layers. Around this corestrand, 8 strands are twisted, each of these strands comprising a corefilament of diameter 0.16 mm, around which 6 filaments of 0.15 mm aretwisted. As an other example, a cord with cord construction(0.25+6*0.23)+6*(0.23+6*0.21) or (0.25+6*0.23)+6*(0.22+6*0.22) may beused. As a further example, a cord with construction(7*0.22)+6*(0.22+6*0.21) or (7*0.22)+6*(0.21+6*0.19) may be used. It isunderstood that each strand may comprise filaments, which are of e.g. adifferent diameter, a different alloy or have a different coating.

A metal cord as subject of the invention has an optical cord diameterpreferably less than 14 mm, e.g. less than 12 mm or less than 9 mm oreven less than 7 mm, such as less than 5 mm, or less than 4 mm or evenless than 2.5 mm, such as less than 2 mm or less than 1.7 mm. Theoptical cord diameter is the diameter of the smallest imaginary circle,which encompasses the radial cross-section of the cord.

The strands used to provide a metal cord as subject of the inventionhave an optical strand diameter preferably less than 5 mm, e.g. lessthan 4 mm or less than 2.5 mm or even less than 1.75 mm, such as lessthan 0.6 mm, The optical strand diameter is the diameter of the smallestimaginary circle, which encompasses the radial cross-section of thestrand.

The diameters of the metal filaments is preferably less than 1.15 mm,e.g. less than 1.05 mm or less than 0.85 mm or even less than 0.7 mm,such as less than 0.5 mm, or less than 0.3 mm or even less than 0.25 mm,such as less than 0.21 mm. Usually however, the diameter of the metalfilaments is not smaller than 0.04 mm.

Metal cord as subject of the invention, preferably are provided out ofsteel alloys, most preferably out of carbon steels, having more than0.275% C or more than 0.4% C or even more than 0.6% C, or stainlesssteel alloys.

A metal cord as subject of the invention has the advantage that the weldon strand level, does not provide an inflexible zone to the metal cord,which, in case such zone would be present, disturbs the use (e.g.bending) of the cord or the product in which the cord is used.

Further, since preferably only a few, or even only one strand is welded,for each radial cross-section of the metal cord, the cord is notweakened too much. The tensile strength of the cord, at the location ofthe welded strand, approaches minimally the sum of the tensile strengthsof the non-welded metal strands in the metal cord. The force at ruptureof the metal cord as subject of the invention comprising welded strandis usually more than 50%, e.g. more than 60% of the force at rupture ofan identical metal cord without strand welds.

The elongation at rupture of the metal cord as subject of the inventioncomprising a welded strand is usually more than 50% e.g. more than 60%or even more than 70% such as more than 80%, e.g. more than 90% of theelongation at rupture of an identical metal cord without welded perradial cross-section or over its welded part length. Since usually highsecurity factors for cord loads are used, the presence of one or a fewstrand welds does not cause any additional risk. The elongation atrupture of the metal cord comprising one or a few strand welds is largercompared to the elongation at rupture of a metal cord being welded overthe whole cross-section of the cord.

Since the metal cords as subject of the invention, overcomes thepresently known technical drawbacks, the cord manufacturer is also ableto reduce the waste-level, and the cords can be provided in a moreeconomical way.

Therefor metal cord as subject of the invention may e.g. be used toreinforce polymer, such as polyurethane, or rubber belts, such as timingbelts, elevator or hoisting belts, conveyor belts, V-belts eitherwithout ribs or with ribs in longitudinal or perpendicular direction, orcan be used for hoisting cables, control cables or in several automotiveapplications such as tires or window elevator cable. Due to the improvedforce at rupture of the metal cord as subject of the invention, the riskon a cascade effect can be decreased over a larger load range, ascompared to cords being welded over the whole cross-section.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described into more detail with reference tothe accompanying drawings wherein

FIG. 1 shows schematically a metal cord as subject of the invention;

FIG. 2 shows schematically a radial cross-section according to the planeAA′ of the metal cord of FIG. 1.

FIGS. 3 and 4 are load-elongation curves of the cord and used strands ofFIG. 1.

FIG. 5 shows a chart of a method to provide a metal cord as subject ofthe invention;

FIG. 6 shows a polymer tape, comprising more than one metal cords assubject of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

An embodiment of a metal cord as subject of the invention is shown inFIG. 1. A metal cord 100, provided out of steel having 0.7% C, having aconstruction “(0.21+6×0.19)+6×(0.19+6×0.175)”. The cord comprises 7strands (101 a, 101 b, 101 c, 101 d, 101 e, 101 f and 101 g). The corestrand 101 a comprising a core filament 102 of diameter 0.21 mm, beingencircled by 6 filaments 103 of a diameter 0.19 mm. The other outerstrands 101 b, 101 c, 101 d, 101 e, 101 f and 101 g having all a corefilament 102 of diameter 0.19 mm, and 6 encircling filaments 103 of0.175 mm.

The lay length 104 of the metal cord 101 is 12.5 mm. The core strand 101a has a lay length of 6.3 mm and the outer strands 101 b, 101 c, 101 d,101 e, 101 f and 101 g having all a lay length of 8 mm. The core strand101 a and the metal cord itself are twisted in the same direction (S orZ direction) whereas the strands 101 b, 101 c, 101 d, 101 e, 101 f and101 g all being twined in the opposite direction (Z, respectively S).

As shown in FIG. 2, being a radial section of the metal cord of FIG. 1according to the plane AA′, strand 101 d has a weld 105, whereas theother strands (101 a, 101 b, 101 c, 101 e, 101 f and 101 g) are notwelded in this section. As shown in FIG. 1, adjacent to both sides ofthe weld 105, strand 101 d has a zone or length 106, where the strandhas a significantly higher torsion.

Where outer strands normally have lay length of 8 mm, in zones 106,strand 101 d has a lay length of 5 mm.

Strand 101 d is incorporated in the metal cord using cabling operations.define a welded part length 107 of the cord, which is for the presentembodiment equal to the lay length of the metal cord. In this zonedetermined by length 107, the strands 101 a, 101 b, 101 c, 101 e, 101 fand 101 g have no weld. The diameter of the weld is essentially equal tothe optical strand diameter, being 0.54 mm

FIGS. 3 and 4 show load-elongation curves of strands and cords, havingin abscissa the elongation, expressed in %, and in ordinate the forceapplied (expressed in Newton)

FIG. 3 shows a load-elongation curve 31 of a strand 101 d of FIG. 1,having a weld, compared to a load-elongation curve 32 of a referencestrand, not being welded. It is noted that the elongation at rupture ofa welded strand of curve 31 is 41.8% of the elongation at rupture of thereference strand.

FIG. 4 clearly shows that a metal cord as in FIG. 1 and FIG. 2,comprising one strand with a weld, of which the load-elongation curve isshown in FIG. 3, has a load-elongation curve 41 which is essentiallyidentical to a load-elongation curve 42 of a similar metal cord, notcomprising a weld. The elongation at rupture of the strand-welded cord(curve 41) is 69% of the elongation at rupture of a not-welded metalcord, whereas the force at rupture of the strand-welded metal cord iseven 90% of the not-welded metal cord.

A method to provide a metal cord as subject of the invention isschematically shown in FIG. 5. E.g. a strand may break or the spool,providing such strand may run empty. Or a filament used to provide astrand may break during production of the strand. In such cases,according to the invention, two ends of a strand are to be connected toeach other.

In step 501, both ends 51 are given a significantly lower lay length,e.g. lay length of 5 mm in stead of the original 8 mm., over a length52, e.g. being 60 mm.

In step 502, at the outer surface of ends 51, the ends are given anessentially flat outer cross-section 54.

Two welding electrodes 55 are fixed one to each end 51 at a distance 56to the cross-section 53, smaller than the length 52 (step 503).

During the welding step 504, both ends 51 are brought in connection toeach other at the cross-sections 53, using a preset pressure. Theelectrodes 55 provide a specified current through the strand for apreset time, which causes welding of the strand.

The weld is than annealed during annealing step 505. Two annealingelectrodes 56 are fixed to the strand, one at each side of the weld. Thedistance 57 between the annealing electrode and the middle of the weldis preferably smaller than the length 52. Possibly, but not necessarily,the welding electrodes may be used as annealing electrodes. During apreset time, a preset current is flowing through the weld.

The welded strand is than further used in cabling or bunching operation506 in order to provide a metal cord 58 as subject of the invention.

Such metal cord as subject of the invention can be used to provide abelt, comprising several metal cords as subject of the invention and apolymer matrix, e.g. polyurethane.

As shown in FIG. 6, metal cords 61 as subjects of the invention areembedded in a polyurethane matrix 62. The metal cord 61 a has a weld 63in one of its strands, used to provide the cord 61 a. Such belt can beused as elevator belt or for several other purposes.

1. A metal cord comprising at least two metal strands, characterized inthat at least one strand is welded, said welded strand having anelongation at rupture of more than 30% of the elongation at rupture ofan identical strand without weld.
 2. A metal cord as in claim 1, saidwelded strand having a force at rupture of more than 30% of the force atrupture of an identical strand without weld.
 3. A metal cord as in claim1, wherein the diameter at the weld of said welded strand is less than1.2 times the optical strand diameter of said welded strand.
 4. A metalcord as in claim 1, wherein for each radial cross-section of said metalcord, at least one of said metal strands is not welded.
 5. A metal cordas in claim 1, wherein for each radial cross-section of said metal cord,only one metal strand is welded.
 6. A metal cord as in claim 5, saidmetal cord having a welded part with a welded part length, the other ofsaid strands being not welded in said welded part, said welded partbeing at least 2.5 times the lay length of said metal cord.
 7. A metalcord as in claim 1, said metal cord having an optical cord diameter lessthan 14 mm.
 8. A metal cord as in claim 7, said metal cord having anoptical cord diameter less than 2.5 mm.
 9. A metal cord as in claim 1,said metal strands having an optical strand diameter less than 5 mm. 10.A metal cord as in claim 9, said metal strands having an optical stranddiameter less than 0.9 mm.
 11. A metal cord as in claim 1, said metalstrands comprising metal filaments, said filaments having a diametersmaller than 1.15 mm.
 12. A metal cord as in claim 11, said metalstrands comprising metal filaments, said filaments having a diametersmaller than 0.25 mm.
 13. A metal cord as in claim 1, said metal cordbeing a steel cord.
 14. A metal cord as in claim 1, said metal cordhaving a force at rupture of more than 50% of an identical metal cordwithout a welded strand.
 15. A metal cord as in claim 1, said metal cordhaving a elongation at rupture of more than 80% of an identical metalcord without a welded strand.
 16. Use of a metal cord as in claim 1, forreinforcing polymer or rubber belts.
 17. Use of a metal cord as in claim1, in elevator or hoisting belts.
 18. Use of a metal cord as in claim 1,in timing belts.
 19. Use of a metal cord as in claim 1, as hoistingcable or control cable or window elevator cable.